This invention relates in general to radio frequency (RF) synthesizers and particularly to RF synthesizers which have high frequency resolutions and which minimize the deleterious effects of noise.
Modern high frequency communication systems such as transceivers or equipment for testing such systems often include frequency converters having synthesizer circuits which perform a variety of functions. For instance, such synthesizers when operated in a "receive" mode provide a local oscillator output signal having a selected one of any of a plurality of predetermined stable reference frequencies. The local oscillator signal can be applied to a mixer to convert the frequencies of a modulated signal. When operated in a "transmit" mode or as a signal generator, the synthesizer provides a selected one of a plurality stable reference frequencies which can be modulated and then either transmitted by an antenna or directly applied to a receiver being tested, for instance.
Some prior art synthesizers each commonly has a phase locked loop (PLL) which includes a phase detector having an output coupled through a loop filter to control the frequency of a voltage controlled oscillator (VCO). The output of the VCO is fed back to one input of the phase detector through a divide-by-N circuit. A constant reference frequency is applied to another input of the phase detector by a crystal oscillator, for instance. The frequency of the VCO output signal is changed in steps by changing the divisor "N" of the divide-by-N circuit in a known manner. This prior art synthesizer is capable of providing N different discrete frequencies which are each separated from the nearest discrete frequency by the frequency of the reference oscillator. Each discrete output frequency is equal to N times the reference frequency.
In many applications, it is desirable for "N" to be kept to a minimum so that the reference frequency can have a high magnitude for a given output frequency. For example, the loop filter is required to attenuate the reference frequency so that it doesn't modulate the VCO. Thus, a high reference frequency enables the loop filter to have a wide bandwidth thereby facilitating rapid change between synthesizer output frequencies responsive to "N" being changed. Additionally, the high reference frequency simplifies the loop filter design. Furthermore, since "N" is a multiplier of the noise generated by the active components within the PLL, it is advantageous from a low noise standpoint for "N" to be minimized. Hence, many advantages result from reducing the magnitude of "N".
The "frequency resolution" of the above-described phase locked loop however is related to the number of separate discrete frequencies provided over a given output bandwidth. Unfortunately, as "N" decreases, the amount of resolution decreases because the number of possible output frequencies is equal to the number of possible values of "N". Changing the value of "N" in the aforementioned low "N" prior art PLL provides a coarse tuning control of the output frequency thereof. For many applications, it is desirable to enable "fine tuning" of the frequency of the PLL output signal to more thoroughly cover a frequency band by providing output signals having smaller predetermined incremental steps therebetween, for instance. Having a high reference frequency complicates meeting this "fine tuning" requirement. More particularly, merely increasing "N" and reducing the reference frequency would decrease the allowable bandwidth of the loop filter thereby increasing acquisition time, complicating the loop filter design and increasing the noise multiplication. Increased noise adversely affects the stability of the synthesizer output frequency.
Some prior art frequency synthesizers having low noise, fast lock and high resolution are too expensive, unreliable, range limited and/or complex for many applications. Also, many such prior art synthesizers require too much electrical power for battery operated equipment. More specifically, prior art solutions for fulfilling the foregoing requirements sometimes use direct frequency synthesizers which have separate mixers, dividers and filters for each decade of output frequency. These circuits are complex and expensive. Other prior art techniques employ indirect frequency synthesizers with multiple loops and RF offset mixing. Additionally, direct digital synthesis, which is employed by numerically controlled oscillators, provide undesirably high spurious and minimum noise floors as well as low operating frequencies. Also, the performance of such direct digital systems is often limited by high speed, digital-to-analog converter circuitry included therein. Moreover, numerically controlled oscillators consume increasing amounts of electrical power as the operating frequency is increased. Still other prior art techniques utilize differential loop synthesis with reference frequencies differing by the desired frequency step size. These frequencies are then mixed to provide the RF offset frequency. Differential loop synthesizers suffer from a limited range of frequency coverage. Also, the above and other prior art techniques sometimes require expensive discrete components for operation at high RF frequencies and careful shielding to minimize spurious